UPDATE 4: I figured out a reasonable way to activate the UI extension app in the Android Emulator and wanted to share. I created a jekyll project on my laptop, created a file with an href link to the app URL that the extension QR code points to, and then ran bundle exec jekyll serve --host=<my local IP>. Then from the Chrome browser in the Android emulator, I visited :4000, clicked on the href link, and got the extension installed.
However, I’m still not able to run the extension UI. I believe the issue is related to https://community.shopify.com/post/2551868 but haven’t gotten it resolved yet.